The present invention relates to an output waveform control circuit of a time division multiple access (TDMA) system, and more particularly to the control of the output waveform of the power amplifier of a transmitter in a TDMA system.
In a TDMA system, in order to prevent the expansion of the spectrum, resulting from the burst switching of the output carrier of a transmitter, from adversely affecting any other transmission channel or the reception channel of the transmitter's own station, the leading and trailing edges of the output signal should be so controlled as to form gentle slopes.
A conventional output waveform control circuit which will be described later referring to FIG. 1 of the accompanying drawings, is disclosed in U.S. Pat. No. 4,706,262 issued to K. Ohta, Nov. 10, 1987 and assigned to the present assignee. In the disclosed circuit, an envelope generating circuit generates a signal whose leading and trailing edges are gently sloped. This signal is then compared by a comparator circuit with a detection signal to provide through a power amplifier an error signal. Next, the error signal supplied by the comparator circuit is applied via a driver circuit to an input level control circuit as a control signal. Responsive to the control signal, the error signal from the power amplifier is so controlled as to become similar to the signal waveform from the envelope detector circuit. In addition, a switching circuit on-off switches the output of a modulator to provide through a frequency converter the switched output to the input level control circuit. The on-off switching at the switching circuit causes the frequency spectrum of the modulator output to be restricted to allow the ON/OFF ratio of the output to be increased.
Since the conventional waveform control circuit smoothes only the leading and trailing edges of the modulator output, the output waveform may still be distorted. This can be due to manufacturing variations of the power amplifier. The distortions on the output waveform cause spurious signals which adversely affect other transmission channels of neighboring base stations.
Furthermore, the rising and falling time of an output in the conventional system may exceed a time period of, for example, 28 microseconds, the standard required for an output with an ON/OFF ratio of 70 dB, under the Groupe Special Mobile (GSM) pan-European mobile telephone system due for launch in July 1991.
Another drawback of the prior art circuit is that the output of the modulator is only switched on and off by the switching circuit and the resulting waveform has steep rising and falling edges. Such a waveform cannot satisfy the GSM standard requiring an output spectrum to have spurious signals of less than 60 dB at f.sub.o .+-.400 KHz and of less than 70 dB at f.sub.o .+-.600 KHz, where f.sub.o is a center frequency.